Fungi II - Phyla Ascomycota and Basidiomycota

Introduction and Goals

This second and final tutorial on fungi will continue our discussion of fungal diversity by covering the two remaining phyla, Ascomycota and Basidiomycota. By the end of this tutorial you should have a basic understanding of:

The life cycle of Phylum Ascomycota

The life cycle of Phylum Basidiomycota

The basic morphology of an ascomycete

The basic morphology of a basdiomycete

Performance Objectives(these are the same for Tutorials 16 and 17):

Identify the general characteristics of the fungi

Explain how the “body” of the fungus is adapted for its mode of nutrition

Diagram the general fungal life cycle and describe the difference between plasmogamy and karyogamy

Summarize the basic classification of fungi

Describe the different types of symbiotic relationships that fungi participate in

For each taxonomic group of fungi, identify the major characteristic(s) of that group and be able to discuss representatives that demonstrate the diversity of the group

Explain different ways in which fungi impact humans, either directly or indirectly

Phylum Ascomycota: General Biology and Morphology

Our discussion of fungal diversity resumes with the phylum Ascomycota. With over 60,000 described species, it is the largest, most biologically and morphologically diverse group in the kingdom. It houses some of the most destructive human and plant pathogens, includes many that form various mutualistic symbioses, and ranges morphologically from unicellular yeasts to complex cup fungi. Almost half of all members of the phylum Ascomycota form symbiotic associations with algae to form lichens (described here under "Environmental Impact"). Others, such as morels (a highly prized edible fungi), form important mychorrhizal relationships with plants, thereby providing enhanced water and nutrient uptake and, in some cases, protection from insects. Thus, in a typical forest, ascomycetes (Fig. 1) can visually manifest themselves as everything from a conspicuous mushroom to disease symptoms on the surface of plant leaves. This group also houses single-celled yeasts. These members of the Ascomycota have been exploited by humans for centuries for the production of fermented foods and beverages (go here for a review of fermentation). Visit this site to see some photos of ascomycetes that are tree pathogenes.

Figure 1. An ascomycete. (Click image to enlarge)

Basic Morphology

The defining morphological character of the phylum Ascomycota is the production of four to eight sexual spores in a microscopic sac-like cell called an ascus, an image of which is shown in Figure 2. Hence, they are sometimes referred to as "sac fungi." In addition, most ascomycetes bear their asci in macroscopic fruiting bodies called ascocarps. Ascomycetes are also capable of producing enormous amounts of asexual spores called conidia, which allow them to propagate without having to undergo sexual recombination. This feature can be particularly destructive because these plant pathogenic fungi can cause devastating epidemics via repeated rounds of asexual reproduction with the dissemination of billions of conidia in a short period of time. Conidia are usually produced externally on the tips of modified hyphae in simple chains or clusters. Prior to sexual reproduction, compatible haploid mating-type hyphae (+ and -) fuse to form a dikaryotic hypha. In contrast to the basidiomycetes, ascomycetes have a more limited dikaryotic stage. The dikaryotic stage eventually gives rise to an ascocarp and sexual ascospores.

Figure 2. Asci of an ascomycete. (Click image to enlarge)

Phylum Ascomycota: Life Cycle

Let's turn our attention to the life cycle of the typical ascomycete depicted in Figure 3. We will start with step 1 in the sexual part of the life cycle, in which two compatible haploid hyphae become intertwined and form an ascogonium and an antheridium (not to be confused with the male gametangium known as an antheridium in plants). In this case, in step 2, the ascogonium acts as a "female" and accepts nuclei from the antheridium after plasmogamy has occurred. In step 3, the resultant dikaryon is then capable of forming a cup-shaped ascocarp. In step 4, asci begin to form on the surface of the ascocarp at the tips of the dikaryotic mycelium, and in step 5, karyogamy occurs to form the highly transient diploid nucleus. In step 6, the diploid nucleus immediately undergoes meiosis, yielding four, genetically distinct, haploid nuclei. In step 7, after an additional round of mitosis, the ascus now contains eight haploid nuclei. In step 8, these eight nuclei will eventually develop into eight ascospores, which are released from the ascus on the surface of the ascocarp. In the final step in the sexual cycle (step 9), haploid mycelia arise from the aforementioned ascospores as the sexual cycle begins again.

Figure 3. An ascomycete life cycle. (Click image to enlarge)

Next, turn your attention to the left side of the diagram. Step 10 depicts the asexual part of the life cycle. Here, a compatible haploid partner is not present and the haploid mycelium is capable of producing asexual spores (conidia) by segmentation of its hyphae. These segments will compartmentalize into conidia, and wind or water dispersal will follow.

Phylum Basidiomycota: Introduction and General Morphology

The fourth, and final, division in the kingdom Fungi that we will cover is the phylum Basidiomycota (Fig. 4). This is the phylum that you are probably most familiar with because it contains fungi which are generally referred to as gilled fungi or gilled mushrooms. However, with over 25,000 classified species, it also houses diverse members such as puffballs, shelf fungi, and rusts (which are important plant pathogens). Basidiomycetes are often called club fungi because the cells (basidia) that bear the sexual spores resemble a small club. Biologically, basidiomycetes follow the same theme as the rest of the fungal kingdom; they are important decomposers, plant pathogens, and symbionts with plants (mycorrhizal). In particular, basidomycetes excel at breaking down large plant cell wall polymers (e.g., lignin found in decaying wood). Morphologically, they range in complexity from microscopic single cells to conspicuous fruiting bodies called basidiocarps. A mushroom that you are likely to see in a forest or at the grocery store is a basidiocarp. Produced during its sexual cycle, it can bear millions of spores on club-shaped basidia located on the surface of its gills. The production of gills underneath the cap is an adaptation that provides a large surface area for spore production. Amazingly, the surface areas on the gills of a store-bought button mushroom could cover the surface of a large trash can lid. This is an example of a highly evolved, morphological feature of gilled fungi. The gills are protected by the top of the cap and elevated by the stem of the basidiocarp for efficient wind dispersal of its basidiospores. Basidiomycetes have also developed unique methods for spore dispersal. Some species produce their spores on basidia in a slimy mass that emits an odor similar to rotting carrion. Flies are attracted to the smell and, in doing so, become agents of spore dispersal.

Figure 4. A basidiomycete fungus. (Click image to enlarge)

Phylum Basidiomycota: Life Cycle

Although some basidiomycetes produce asexual spores, asexual reproduction is far more common in the phylum Ascomycota. Therefore, we will discuss a generalized life cycle covering sexual reproduction. Turn your attention to the basidiomycete life cycle in Figure 5.

Figure 5. A basidiomycete life cycle. (Click image to enlarge)

In steps 1 and 2, two haploid hyphae of opposite mating types fuse to produce a dikaryotic hypha. In step 3, under favorable environmental conditions (optimum temperature and moisture levels), the dikaryotic hypha is capable of producing a fruitbody; in this case, a gilled mushroom. In step 4, basidia begin to form on the surface of the gills, and in step 5, karyogamy occurs to form a diploid nucleus in each basidium. In step 6, meiosis immediately follows karyogamy and each of four genetically distinct, haploid nuclei migrates into appendages and develops into basidiospores. Basidiospores are released and wind dispersed. They germinate into haploid hyphae and the cycle begins again in step 7.

Summary

This concludes our discussion on the kingdom Fungi. You should now have a basic understanding of the general biology of the four phyla that comprise the kingdom, including ecological roles, basic biology, reproductive features, and fungal diversity. Proceed on to the review questions and be sure to review sections where you need to refamiliarize yourself with the material.

Terms

After reading this tutorial, you should have a working knowledge of the following terms: